Three-piece, wound balls with balata (trans-polyisoprene) covers are typically preferred by professional and low handicap amateur golfers. These balls provide a combination of distance, high spin rate, and control that is not available with an ionomer cover or in one-piece and two-piece balls. However, balata cuts easily, and lacks the durability required by the average golfer.
Two-piece golf balls, which are typically used by the average amateur golfer, provide a combination of durability and maximum distance that is not available with balata covered balls. These balls comprise a core, formed of a solid sphere which typically comprises a polybutadiene based compound, encased in an ionomer cover formed of, e.g., SURLYN.RTM.. These ionomers are ionic copolymers of an olefin and an unsaturated carboxylic acid in which at least a portion of the carboxylic acid groups have been neutralized with a metal ion. These balls are extremely durable, have good shear resistance and are almost impossible to cut. However, the durability results from the hardness of the ionomer, which gives such balls a very hard "feel" when struck with a golf club that many golfers find unacceptable.
Golf ball manufacturers have attempted to produce golf ball covers that provide the spin rate of balata with the cut resistance of an ionomer by forming blends of high hardness and low hardness ionomers, e.g., U.S. Pat. Nos. 4,884,814, 5,120,791, 5,324,783 and 5,492,972. However, none of the disclosed ionomer blends have resulted in the ideal balance of carrying distance, coefficient of restitution, spin rate and initial velocity that would approach the highly-desirable playability of a balata covered golf ball. This approach is exemplified in U.S. Pat. No. 5,415,937 to Cadorniga et al. Cadorniga et al. disclose a golf ball cover material consisting of a blend of a high stiffness ionomer, preferably with a Shore D hardness of at least 70 and a flexural modulus of 60,000 to 120,000 psi, and a very low modulus ionomer, preferably with a Shore D hardness of 20 to 50 and a flexural modulus of 2,000 to 8,000 psi. The purpose is to improve the feel and playability of the ball when compared to a standard ionomer cover, while retaining the distance and resilience of the prior art balls. Golf balls having covers incorporating the disclosed blends have a slightly improved coefficient of restitution and initial velocity with spin rates that range from slightly better than prior art blends to significantly lower, depending upon the particular blend and the club used in the test, i.e., driver, 5-iron, or pitching wedge.
Manufacturers have also attempted to form blends of hard ionomers with softer, nonionomer polymers to soften the golf ball and improve its feel and spin rate. However, this approach has proven to be difficult because the ionic character of ionomers imparts a highly polar nature to these materials. Therefore, ionomers and other nonionomer polymers, such as balata, and polyolefin homopolymers, copolymers, or terpolymers that do not contain ionic, acidic, basic, or other polar pendant groups, have not been successfully blended for use in golf ball covers. These mixtures often have poor mechanical properties such as inferior tensile strength, impact strength, and the like. Hence, the golf balls produced from these immiscible mixtures will have inferior golf ball properties such as poor durability and cut resistance on impact.
Adding polar functionality to nonpolar polymers is another approach which has been used to facilitate the blending of nonionomers with ionomers for golf ball cover materials. For example, U.S. Pat. Nos. 4,986,545, 5,098,105 and 5,359,000 all disclose compatible or miscible blends between ionomers and another polymer. Compatibility is accomplished by imparting polar functionality to the nonionomer through a reaction with maleic anhydride. None of these patents, however, discloses blends of nonionomer polymers with polyamides.
Because of the difficulties encountered when attempting to blend ionomers with other polymers, manufacturers have used compatibilizers to provide or enhance the compatible nature of such blends; see, for example, U.S. Pat. No. 5,321,089. The compatibilizer material is often a block copolymer where each block has an affinity for only one of the blend components to be compatibilized. The compatibilizer is thought to associate across the boundaries between phase-separated regions in the polymer blend. It is used to bind the regions together and to enhance the structural integrity and mechanical properties of the resulting compatibilized material.
U.S. Pat. No. 5,155,157 to Statz et al. describes thermoplastic elastomer (hereafter "TPE") compositions that are blends of a copoly(ether-amide) with an acid-containing ethylene copolymer ionomer and an epoxy containing compound, for use in one-piece golf balls and as cores for two-piece and three-piece golf balls. Japanese patent application 6192512 A (1994) discloses compositions which are blends of a thermoplastic polyamide elastomer, an ethylene copolymer ionomer and an epoxy-containing compound for use in two-piece and three-piece golf ball covers and cores. In each of these disclosures or publications, a costly custom-synthesized compatibilizer component is required to compatibilize a blend of one or more ionomers with a polymer that is immiscible with the ionomer. None of the above disclosures or publications teaches a blend of a nonionomer polymer with a polyamide.
Two-piece golf balls having covers containing block polyamide copolymers are disclosed in the prior art. For example, U.S. Pat. No. 4,234,184 to Deleens et al. discloses the use of a thermoplastic block copoly(ether-amide) as a cover material for a golf ball having a core and a cover. Deleens et al. also disclose blends of this block copolymer with minor proportions of compatible polymer(s) which are further required to have a melting point between 80.degree. and 150.degree. C. and a Shore D hardness from 35 to 70. Blends of this block copolymer with polyamide are not disclosed.
Several patents disclose blends of polyamide elastomers and ionomers. For example, U.S. Pat. No. 4,858,924 to Saito discloses the use of a thermoplastic resin with a flexural modulus of 1,500 to 5,000 kg/cm.sup.2 as the cover of a golf ball. Particularly, polyamide elastomer, urethane elastomer, styrene-butadiene copolymer elastomer and polyester elastomer are said to be preferred when used alone or blended with a matrix resin, that is, another like flexible thermoplastic resin. The polyester elastomers are said to include block copoly(ether-esters), block copoly(lactone-esters) and aliphatic and aromatic dicarboxylic acid copolymerized polyesters. However, this reference does not teach that polyamide can be a matrix resin.
Multilayer golf balls containing block copolymers are disclosed in the prior art. For example, pertaining to covers, UK Patent Application GB 2,278,609 A discloses a three-piece golf ball with an outer or cover layer formed from a relatively soft, low modulus (1 to 10 kpsi) nonionomer TPE, such as a polyurethane (ESTANE.RTM. from B. F. Goodrich, TEXIN.RTM. from Bayer and PELLETHANE.RTM. from Dow are taught), a polyester elastomer (HYTREL.RTM. from DuPont is taught), or a polyester amide (PEBAX.RTM. from Elf Atochem S. A. is taught). Blends of these materials with polyamide are not disclosed.
Intermediate layers containing block copolymers are disclosed for multilayer golf balls. For example, U.S. Pat. No. 5,556,098 to Higuchi et al. discloses the use of a three-layer golf ball with a soft middle layer composed of a blend of a polyamide elastomer and an ionomer, such that the JIS C hardness of the blend is less than 80. The exact chemical composition or structure of the polyamide elastomer is not disclosed other than that it is said to be a thermoplastic elastomer. Higuchi et al. are silent on the flexural modulus characteristics of these blends and of their components. Furthermore, Higuchi does not disclose blends of these elastomers with polyamide.
U.S. Pat. No. 5,253,871 to Viollaz discloses the use of at least 10% of a block copoly(amide-ether) elastomer, optionally blended with an ionomer, for use as the middle layer of a three-layer golf ball. The hardness of the block copolymer is said to be within the range of 30-40 Shore D hardness while the corresponding hardness of the ionomer component is said to be between 55-65 Shore D. The overall hardness of the middle layer is said to range from 20-50 Shore D. The cover may also be a block copoly(amide-ether) and ionomer blend but its overall hardness must be greater than that of the adjacent middle layer. However, Viollaz is silent on the flexural modulus characteristics of the blends or their components. Furthermore, Viollaz does not disclose blends of these block copolymers with polyamide.
Australian patent publication No. AU-A-60631/96 discloses the use of a polyamide polymer in golf balls, but only in a three-piece golf ball. The teachings of this reference are further limited in many respects. For example, the polyamide must be present only in the intermediate layer of the three-piece golf ball and then present only in the form of a blend with certain thermoplastic elastomers. Moreover, the reference teaches that the blend comprises only 50% to 95% polyamide by weight. Styrene-butadiene-styrene block copolymer, maleic anhydride-modified styrene-butadiene-styrene block copolymer, ethylene-ethyl acrylate copolymer, and maleic anhydride-modified ethylene-ethyl acrylate copolymer are the only thermoplastic elastomers disclosed for blending with the polyamide. Furthermore, the reference teaches that these four thermoplastic elastomers must be within the JIS-A hardness range of 30 to 98. Even further, the polyamide blended with these thermoplastic elastomers is taught to have a flexural modulus between 6,000 and 30,000 kg/cm.sup.2 (85 and 427 kpsi). Additionally, the resulting blended composition is disclosed to have a flexural modulus of between only 5,000 and 12,000 kg/cm.sup.2 (71 and 171 kpsi).
U.S. Pat. No. 4,679,795 to Melvin et al. discloses blends of optical brighteners with the following golf ball cover materials: polyolefins and their copolymers; polyurethanes; polyamides; polyamide blends with SURLYN.RTM., polyethylene, ethylene copolymers and EPDM; acrylic resins; thermoplastic rubbers such as urethanes, styrene block copolymers, copoly(ether-amides) and olefinic thermoplastic rubbers; thermoplastic polyesters and polyester TPEs; and blends of thermoplastic rubbers with nylon. The reference contains no teaching or suggestion, however, to form the blend without the required optical brightener component.
None of the blended compositions described above offers the combination of durability and distance provided by two-piece golf balls with ionomer covers and the high spin rate and control that is available with three-piece, wound golf balls having balata covers. Therefore, there remains a need for golf ball cores, intermediate layers and covers that comprise a polyamide, optionally blended with a nonionomer polymer, to provide one-piece, two-piece and/or multilayer golf balls with the durability and distance of a SURLYN.RTM. covered two-piece ball and the feel, click, and control of a balata covered three-piece ball.